The invention relates to a process for melting down sponge metal, in particular sponge iron, in an electric-arc furnace having at least one electrode, wherein the sponge metal is introduced into the electric-arc furnace in the form of at least one sponge metal jet which in the immediate vicinity of an electrode hits the bath level present in the electric-arc furnace and wherein for decarburization and/or intermixing the bath and/or charging energy into the metal melt, oxygen is blown into the melt, and the invention relates to an electric-arc furnace for carrying out the process.
It is known (EP-0 964 065 A1, EP-0 418 656 A1) to introduce solids, such as steel works dusts, untreated filter dusts and/or scale or unreduced ore, into an electric-arc furnace, namely to blow them into a metal melt present in the electric-arc furnace. Here, it has turned out to be advantageous to supply, via a lance, oxygen above the bath level of the metal melt and within a slag layer situated above it and to thereby carry out in a region adjacent to the metal melt a post-combustion of carbon monoxide formed by a separate O2 supply and to charge the heat quantity obtained in that post-combustion immediately into the metal melt.
Furthermore, it is known to provide post-combustion means for applying the CO post-combustion technology when smelting scrap. For this purpose, post-combustion nozzles and/or burners are fixedly installed in the electric-arc furnace. However, such means can not be used under flat-bath conditions.
The invention has as its object to make it possible to apply the CO post-combustion technology also to electric-arc furnaces which serve the purpose of melting down sponge metal, in particular sponge iron, under flat-bath conditions. With electric-arc furnaces for melting down sponge iron, sponge iron is introduced through a cover hole, which is arranged outside the center, namely roughly at the circumference of a cover heart, via a chute or slide, and this in large quantities of up to 7,000 kg/min. In practice, in the absence of specific measures, an average of 28 to 40 kg of sponge iron is introduced per MW of electric-power input and per minute. The sponge iron, which is in lumpy form (pellets and/or briquets) as well as, optionally, also in fine-particulate form, gets for example in the form of a trajectory parabola close to the center of the electric-arc furnace, that is, its energy center, in which one electrode or several electrodes are arranged. Hereby, a quick melting-down results. With a furnace of that kind, there is the problem that during improper introduction of oxygen for post-combustion purposes there may occur an increased electrode consumption by the oxygen, a decrease in the metallic discharging by increased iron loss as well as an increase in the thermal stress of the wall and cover elements of the electric-arc furnace, which in general are water-cooled, without energy being additionally transferred to the molten metal.
The invention aims at avoiding these disadvantages and difficulties and has as its object to provide a process and an electric-arc furnace which enable a very efficient CO post-combustion when melting down sponge iron under flat-bath conditions, wherein introduction of the sponge metal into the energy center or its immediate vicinity should be possible.
According to the invention, this object is achieved in that in addition to the oxygen introduced for decarburization and/or intermixing the bath and/or charging energy into the metal melt at least one jet of oxygen or one jet of an oxygen-containing gas is blown into the electric-arc furnace at a low rate for a CO post-combustion, which jet hits the bath level in the region of the point of incidence of the sponge metal jet, and the sponge metal jet preferably is conveyed into the electric-arc furnace by gravitation alone, and/or immediately adjacent to the point of incidence of the sponge metal jet and which jet in the region or vicinity of that point of incidence, on the side facing the electrode(s) of the electric-arc furnace, is shielded by the sponge metal jet relative to the electrode(s) in the form of a protective shield.
Hereby, the CO forming during the impact of the carbon-containing sponge iron on the oxygen-containing slag and the subsequent melting-down is subjected to a post-combustion with oxygen to form CO2, and the energy thus being released is additionally imparted to the sponge iron of the feed jet and to the slag and/or the bath. The hot slag thereby exhibits an improved foaming behavior and better envelops the electric arc(s), whereby the radiation of heat to the water-cooled wall and cover elements is reduced. All in all, energy savings of xe2x89xa735 kWh/t of liquid steel result therefrom.
Thus, according to the invention, at least one separate jet of oxygen or jet of an oxygen-containing gas is introduced into the electric-arc furnace only for the CO post-combustion; here, it is essential that this/these oxygen jet(s) is/are directed into the energy center and into the region and/or immediate vicinity of the point of incidence of the sponge iron on the melt and that a protection of the electrodes from direct contact with oxygen is nonetheless provided.
The oxygen jet or jet of an oxygen-containing gas, which is additionally blown in, preferably is blown into the electric-arc furnace at a subsonic speed.
Preferably, the oxygen jet or jet of an oxygen-containing gas, which is additionally blown in, is blown in under a low pressure, preferably under a pressure of 6 bars at the most, so that the slag is not displaced by the oxygen jet or jet of an oxygen-containing gas, additionally introduced for the CO post-combustion, whereby the electric arcs effectively remain enveloped by foamed slag. By a low pressure, the pressure at which no supersonic speed is attained with a predetermined nozzle diameter is understood.
Suitably, the sponge iron is introduced into the electric-arc furnace in lumpy form, preferably as pellets and/or briquets, and, optionally, partly in the form of fines.
According to a preferred embodiment, the oxygen jet or jet of an oxygen-containing gas from roughly half the free height of the interior of the electric-arc furnace to the point of incidence on the bath level is shielded by the sponge metal jet relative to the electrode(s) at least on the side facing the electrode(s), xe2x80x9croughlyxe2x80x9d comprisingxc2x120%, preferablyxc2x110%.
An electric-arc furnace for carrying out the process according to the invention, comprising at least one electrode and a cover charging hole for charging sponge metal by gravitation and comprising an oxygen feeding means for introducing oxygen into the metal melt, is characterized by at least one oxygen lance which through a cover opening can be brought into a position projecting into the inside of the furnace, which position is oriented in such a manner that the oxygen jet, seen from one electrode or the electrodes, at least in the region of the point of incidence of the sponge metal on the bath level is shielded relative to the electrode(s) by the sponge metal falling into the electric-arc furnace by gravitation.
For the exact positioning of the oxygen supply, the oxygen lance is movable relative to the electric-arc furnace, preferably pivotable and forwardly and backwardly displaceable.